1. Field of the Invention
The present invention relates to a surface treatment agent for fine surface treatment and, more particularly, to a surface treatment agent for use in fine surface treatment which is very effective for wet etching of silicon oxide film in the manufacturing process of semiconductor devices, as well as cleaning of fine-treated semiconductor devices.
2. Description of Prior Art
Recently, there has been an increasing demand for cleanness, precision and high technology in the steps of cleaning, etching and patterning of wafer surfaces and fine treatment surfaces in wet process of semiconductor integrated circuits under the progress of high integration. So-called buffered hydrogen fluoride such as hydrofluoric acid, mixed solution of hydrogen fluoride and ammonium fluoride have been used for cleaning, etching and patterning as a surface treatment agent for fine surface treatment, and to meet the requirement for submicron ultra high integration, higher performance and function of the buffered hydrogen fluoride has been increasingly demanded.
Buffered hydrogen fluoride is usually prepared by mixing 40% ammonium fluoride and 50% fluoric acid in various mixing ratios, and is applied to the step of etching while controlling etching speed of silicon oxide film from 90 .ANG./min to 1200 .ANG./min in the range of 100:1 to 6:1, for example. In this case, the chemical composition of the buffered hydrogen fluoride is about 35 to 40% ammonium fluoride and about 0.5 to 7% hydrogen fluoride. It is generally considered that ammonium fluoride is necessary for adjustment of etching speed and as a buffer.
Several methods for improving wettability of such buffered hydrogen fluoride with respect to surface material of wafer have been recently proposed. For example, U.S. Pat. No. 3,650,960 and Japanese Laid-Open Patent Publication (unexamined) No. 63-283028 proposed improvement in uniformity and adhesion of particles in fine etching process by providing wettability by mixing aliphatic carboxylic acid, salt of aliphatic carboxylic acid, aliphatic amine and aliphatic alcohol. In this buffered hydrogen fluoride composition, however, a problem exists in that crystallization may take place during the storage in winter season due to a high concentration of ammonium fluoride. Thus, to overcome such a problem, the foregoing U.S. Pat. No. 3,650,960 and Japanese Laid-Open Patent Publication (unexamined) No. 63-283028 propose also the employment of other compositions having a reduced concentration of ammonium fluoride.
When looking into etching function of buffered hydrogen fluoride acid in detail from the viewpoint of ion chemical reaction mechanism, it is found that the chemical composition employed hitherto is not appropriate from stoichiometric point of view and, moreover, that there is an essential defect of impeding the progress of normal etching reaction therein. Problems in terms of prior chemical compositions can be summarized as follows:
(1) To produce HF.sub.2.sup.- ion which is required for etching of SiO.sub.2 film, HF needs equimolar NH.sub.4 F, and concentration of NH.sub.4 F exceeding the same makes no contribution to etching reaction.
Accordingly, increase in mixing ratio of 40% ammonium fluoride to 50% hydrogen fluoride to achieve low etching ratio will bring about increase in unnecessarily excessive concentration of NH.sub.4 F.
(2) As a result of measuring solubility of ammonium hexafluoro silicate (NH.sub.4).sub.2 SiF.sub.6 which is produced by reaction between SiO.sub.2 film and HF.sub.2.sup.- ion and soluble in buffered hydrogen fluoride solution with respect to various buffered hydrogen fluoride, it was recognized that solubility of this product is reduced significantly due to increase in concentration of NH.sub.4 F.
Accordingly, it may be said that unnecessarily excessive concentration of NH.sub.4 F brings about considerable problems in the aspects of time linearity in fine pattern etching reaction and uniformity in patterning.
(3) Wettability of buffered hydrogen fluoride can be improved by mixing aliphatic hydrocarbon surfactant. There is, however, a limited narrow optimum range in the relation between concentration of surfactant and function thereof. It is an important factor to control the concentration.
When the concentration of NH.sub.4 F is higher, the mentioned optimum range is narrower.
It has been also expected that cleaning function of diluted hydrogen fluoride, when applied to wafer surface or other surface to be finely treated as a cleaner, is considerably improved by giving wettability to the diluted hydrogen fluoride. At present, however, no surfactant satisfying required cleanness, stability and performance has been developed.
Cleaning using diluted fluoric acid is one of the important processes, and in which smoothness of cleaned surface is a very important factor in submicron devices. There remains some dry spots on a silicon wafer surface etched with buffered hydrogen fluoride. It is said that the dry spots can be normalized by mixing a suitable surfactant, as is proposed by Japanese Laid-Open Patent Publication No. 63-283028. However, as a result of analyzing a silicon wafer cleaned with diluted hydrogen fluoride, it was found that smoothness of the cleaned surface was damaged. This kind of damage results in grave problem, for example, in the formation of shallow junction. Accordingly, it seems essential to develop means for achieving smoothness on the surface by providing wettability even in the cleaning step with diluted hydrogen fluoride.